PHOTOGRAPHY

Eclipse Photography

The eclipse may be safely photographed provided that the above precautions are followed. Almost any kind of 35mm camera with manual controls can be used to capture this rare event; however, a lens with a fairly long focal length is recommended to produce as large an image of the Sun as possible. A standard 50mm lens yields a minuscule 0.5mm image, while a 200mm telephoto or zoom produces a 1.9mm image (Figure 24). A better choice would be one of the small, compact catadioptic or mirror lenses that have become widely available in the past 20 years. The focal length of 500mm is most common among such mirror lenses and yields a solar image of 4.6mm. With one solar radius of corona on either side an eclipse view during totality will cover 9.2mm. Adding a 2X teleconverter will produce a 1000mm focal length, which doubles the Sun's size to 9.2mm. Focal lengths in excess of 1000mm usually fall within the realm of amateur telescopes.

If full disk photography of partial phases on 35mm format is planned, the focal length of the optics must not exceed 2600mm. Because most cameras do not show the full extent of the image in their viewfinders, a more practical limit is about 2000mm. Longer focal lengths permit photography of only a magnified portion of the Sun's disk. In order to photograph the Sun's corona during totality, the focal length should be no longer than 1500 - 1800mm (for 35mm equipment); however, a focal length of 1000mm requires less critical framing and can capture some of the longer coronal streamers. Figure24 shows the apparent size of the Sun (or Moon) and the outer corona on a 35mm film frame for a range of lens focal lengths. For any particular focal length, the diameter of the Sun's image is approximately equal to the focal length divided by 109 (Table 22).

Figure 24: Lens Focal Length vs. Image Size for Eclipse Photography

A solar filter must be used on the lens throughout the partial phases for both photography and safe viewing. Such filters are most easily obtained through manufacturers and dealers listed in Sky & Telescope and Astronomy magazine (see Section 3.2, "Sources for Solar Filters"). These filters typically attenuate the Sun's visible and infrared energy by a factor of 100,000. The actual filter factor and choice of ISO film speed, however, will play critical roles in determining the correct photographic exposure. Almost any speed film can be used because the Sun gives off abundant light. The easiest method for determining the correct exposure is accomplished by running a calibration test on the uneclipsed Sun. Shoot a roll of film of the mid-day Sun at a fixed aperture (f/8 to f/16) using every shutter speed from 1/1000s to 1/4s. After the film is developed, note the best exposures and use them to photograph all the partial phases. The Sun's surface brightness remains constant throughout the eclipse, so no exposure compensation is needed except for the narrow crescent phases, which require two more stops due to solar limb darkening. Bracketing by several stops is also necessary if haze or clouds interfere on eclipse day.

Certainly the most spectacular and awe-inspiring phase of the eclipse is totality. For a few brief minutes or seconds, the Sun's pearly white corona, red prominences, and chromosphere are visible. The great challenge is to obtain a set of photographs that captures some aspect of these fleeting phenomena. The most important point to remember is that during the total phase, all solar filters must be removed! The corona has a surface brightness a million times fainter than the photosphere, so photographs of the corona are made without a filter. Furthermore, it is completely safe to view the totally eclipsed Sun directly with the naked eye. No filters are needed, and in fact, they would only hinder the view. The average brightness of the corona varies inversely with the distance from the Sun's limb. The inner corona is far brighter than the outer corona; thus, no single exposure can capture its full dynamic range. The best strategy is to choose one aperture or f/number and bracket the exposures over a range of shutter speeds (i.e., 1/1000s down to 1s). Rehearsing this sequence is highly recommended because great excitement accompanies totality and there is little time to think.

Exposure times for various combinations of film speeds (ISO), apertures (f/number) and solar features (chromosphere, prominences, inner, middle, and outer corona) are summarized in Table 23. The table was developed from eclipse photographs made by F. Espenak, as well as from photographs published in Sky and Telescope . To use the table, first select the ISO film speed in the upper left column. Next, move to the right to the desired aperture or f/number for the chosen ISO. The shutter speeds in that column may be used as starting points for photographing various features and phenomena tabulated in the "Subject" column at the far left. For example, to photograph prominences using ISO 400 at f/16, the table recommends an exposure of 1/1000. Alternatively, the recommended shutter speed can be calculated using the 'Q' factors tabulated along with the exposure formula at the bottom of Table 23. Keep in mind that these exposures are based on a clear sky and a corona of average brightness. The exposures should be bracketed one or more stops to take into account the actual sky conditions and the variable nature of these phenomena.

An interesting, but challenging, way to photograph the eclipse is to record its phases all on one frame. This is accomplished by using a stationary camera capable of making multiple exposures (check the camera instruction manual). Because the Sun moves through the sky at the rate of 15° per hour, it slowly drifts through the field of view of any camera equipped with a normal focal length lens (i.e., 35 - 50 mm). If the camera is oriented so that the Sun drifts along the frame's diagonal, it will take over 3h for the Sun to cross the field of a 50 mm lens. The proper camera orientation can be determined through trial and error several days before the eclipse. This will also ensure that no trees or buildings obscure the view during the eclipse. The Sun should be positioned along the eastern (left in the Northern Hemisphere) edge or corner of the viewfinder shortly before the eclipse begins. Exposures are then made throughout the eclipse at ~5min intervals. The camera must remain perfectly rigid during this period and may be clamped to a wall or post because tripods are easily bumped. If in the path of totality, remove the solar filter during the total phase and take a long exposure (~1s) in order to record the corona in the sequence. The resulting photograph will consist of a string of Suns, each showing a different phase of the eclipse.

Finally, an eclipse effect that is easily captured with point-and-shoot or automatic cameras should not be overlooked. Use a kitchen sieve or colander and allow its shadow to fall on a piece of white cardboard placed several feet away. The holes in the utensil act like pinhole cameras and each one projects its own image of the Sun. The effect can also be duplicated by forming a small aperture with one's hands and watching the ground below. The pinhole camera effect becomes more prominent with increasing eclipse magnitude. Virtually any camera can be used to photograph the phenomenon, but automatic cameras must have their flashes turned off because this would otherwise obliterate the pinhole images.

Several comments apply to those who choose to photograph the eclipse aboard a cruise ship at sea. Shipboard photography puts certain limits on the focal length and shutter speeds that can be used. It is difficult to make specific recommendations because it depends on the stability of the ship, as well as wave heights encountered on eclipse day. Certainly telescopes with focal lengths of 1000 mm or more can be ruled out because their small fields of view would require the ship to remain virtually motionless during totality, and this is rather unlikely even given calm seas. A 500 mm lens might be a safe upper limit in focal length. ISO 400 is a good film speed choice for photography at sea. If it is a calm day, shutter speeds as slow as 1/8 or 1/4 may be tried. Otherwise, use a 1/15 or 1/30 shutter speed and shoot a sequence through 1/1000s. It might be good insurance to bring a wider 200 mm lens just in case the seas are rougher than expected. A worst case scenario is when Espenak photographed the 1984 total eclipse aboard a 95ft yacht in seas with wave heights of 3ft. He had to hold on with one hand and point his 350 mm lens with the other! Even at that short focal length, it was difficult to keep the Sun in the field, however, any large cruise ship will offer a far more stable platform than this. New image stabilizer lenses from Canon and Nikon may also be helpful aboard ship by allowing the use of slower shutter speeds.

Table 23. Solar Eclipse Exposure Guide

ISO		f/Number
25		1.4	2	2.8	4	5.6	8	11	16	22
50		2	2.8	4	5.6	8	11	16	22	32
100		2.8	4	5.6	8	11	16	22	32	44
200		4	5.6	8	11	16	22	32	44	64
400		5.6	8	11	16	22	32	44	64	88
800		8	11	16	22	32	44	64	88	128
1600		11	16	22	32	44	64	88	128	176
Subject	Q	Shutter Speed (s)
Solar Eclipse
Partial[1] - 4.0 ND	11	-	-	-	1/4000	1/2000	1/1000	1/500	1/250	1/125
Partial[1] - 5.0 ND	8	1/4000	1/2000	1/1000	1/500	1/250	1/125	1/60	1/30	1/15
Baily's Beads[2]	12	-	-	-	-	1/4000	1/2000	1/1000	1/500	1/250
Chromosphere	11	-	-	-	1/4000	1/2000	1/1000	1/500	1/250	1/125
Prominences	9	-	1/4000	1/2000	1/1000	1/500	1/250	1/125	1/60	1/30
Corona - 0.1 Rs	7	1/2000	1/1000	1/500	1/250	1/125	1/60	1/30	1/15	1/8
Corona - 0.2 Rs[3]	5	1/500	1/250	1/125	1/60	1/30	1/15	1/8	1/4	1/2
Corona - 0.5 Rs	3	1/125	1/60	1/30	1/15	1/8	1/4	1/2	1	2
Corona - 1.0 Rs	1	1/30	1/15	1/8	1/4	1/2	1	2	4	8
Corona - 2.0 Rs	0	1/15	1/8	1/4	1/2	1	2	4	8	15
Corona - 4.0 Rs	-1	1/8	1/4	1/2	1	2	4	8	15	30
Corona - 8.0 Rs	-3	1/2	1	2	4	8	15	30	60	120

Exposure formula:
t = f 2/(I x 2Q),

where:
t = exposure time (s)
f = f/number or focal ratio
I = ISO film speed
Q = brightness exponent

Abbreviations: ND = Neutral Density Filter. Rs = Solar Radii.

Notes:
[1] Exposures for partial phases are also good for annular eclipses.
[2] Baily's Beads are extremely bright and change rapidly.
[3] This exposure also recommended for the 'Diamond Ring' effect.

Consumer digital cameras have become affordable in recent years and many of these may be used to photograph the eclipse. Most recommendations for 35 mm single lens reflex (SLR) cameras apply to digital SLR (D-SLR) cameras as well. The primary difference is that the imaging chip in many D-SLR cameras is only about 2/3 the area of a 35 mm film frame (see the camera's technical specifications). This means that the Sun's relative size will be about 1.5 times larger in a D-SLR camera so a shorter focal length lens can be used to achieve the same angular coverage compared to a 35 mm SLR camera. Another issue to consider is the lag time between digital frames required to write images to the camera's memory card. It is also advisable to turn off autofocus because it is not reliable under these conditions; focus the camera manually instead. Preparations must be made for adequate battery power and space on the memory card.

From NASA and Fred Espenak Solar eclipse information pages